Prior-art methods of monitoring particles in a stack include both electrical and optical methods. In a typical optical method, a beam of light from a laser source is directed across the stack (which is typically of diameter 0.5 m to 10 m). Particles in the stack scatter the laser beam over all solid angles with an angle-dependent intensity typically enhanced in a direction of scatter around the forward direction of an incident beam. A portion of the scattered light is collected by a simple telescope and focused onto a photo-detector; the portion of light collected varies in different prior-art devices (e.g. because the collected light is from different scattering directions).
In addition to the light scattered from particles flowing in the stack, other objects that are illuminated by the laser will scatter light. Objects that provide such unwanted scatter are usually part of the stack or equipment fitted to the stack, including the monitoring equipment itself (which includes windows, mirrors, beam dumps, and lenses). Light scattered from such objects can be misinterpreted by the monitoring apparatus if the light finds its way onto the detector.
Manufacturers of monitoring apparatus go to some trouble to prevent such stray light from reaching the detector. One example prior-art method of elimination of stray light is to restrict the field of view of the telescope so that the detector does not “see” any bright scatter from the laser window or that part of the laser beam which coincides with a stack wall. That method of exclusion necessitates collection of scattered light which is angularly-separated from the forward direction of propagation of the laser beam and is therefore not optimal in terms of intensity and relative immunity from the effect of particle size distribution. Another example prior-art method is to take images of scattered light from particulate matter illuminated at an angle which is far from the optimum angle. Imaging a scatter field at large scatter angles has the advantage of achieving focus and range resolution over the entire field, but that is achieved at the expense of signal strength, and prevents mounting of transmitter and receivers in convenient cross-diametric stack locations.
The present invention seeks to mitigate the above-mentioned problems.